Evaluation of aquifer potential, geoelectric and hydraulic parameters in Ezza North, southeastern Nigeria, using geoelectric sounding

Transcription

1 Int. J. Environ. Sci. Technol. (2016) 13: DOI /s y ORIGIAL PAPER Evaluation of aquifer potential, geoelectric and hydraulic parameters in Ezza orth, southeastern igeria, using geoelectric sounding D.. Obiora 1 J. C. Ibuot 1. J. George 2 Received: 19 April 2015 / Revised: 13 August 2015 / Accepted: 2 September 2015 / Published online: 14 September 2015 Islamic Azad University (IAU) 2015 Abstract A geoelectric survey involving vertical electrical sounding (VES) employing Schlumberger electrode configuration was carried out with the aim of evaluating the aquifer potential, electric and hydraulic parameters in Ezza orth. Schlumberger electrode configuration was used to acquire data for twelve VES stations. The interpreted and analyzed results reveal four to six geoelectric layers. The VES curves obtained were QQH, QHK, QHA, QQQ, HAK, KHK, HKH and QQ. From the result, the Dar Zarrouk parameters (longitudinal conductance and transverse resistance) were calculated. The longitudinal conductance ranges between and 4.6 mhos. The transverse resistance ranges between and 38,808 Xm 2. The range of hydraulic conductivity is m/day, while the range of transmissivity is m 2 /day from the estimated values. The contour maps were drawn using the electrical and hydraulic parameters, and the distribution of the aquifer parameters is shown. Based on the results, aquifer potential and protective capacity of the study area were determined. Keywords Aquifer potential Ezza orth Hydraulic conductivity Longitudinal conductance Transmissivity Transverse resistance & D.. Obiora daniel.obiora@unn.edu.ng 1 2 Department of Physics and Astronomy, University of igeria, sukka, Enugu State, igeria Department of Physics, Akwa Ibom State University, Ikot Akpaden, Uyo, Akwa Ibom State, igeria Introduction A detailed knowledge of the subsurface geology and structure is provided by the geophysical surveys. Electrical resistivity method has been used successfully in delineation and exploitation of groundwater (Evans et al. 2010; George et al. 2010; Ibuot et al. 2013). It gives detailed information about hydrogeological settings and groundwater repositories. Groundwater is that water contained in the voids of the geologic materials that comprise the crust of the earth and exists at a pressure greater than or equal to atmospheric pressure (Al Sabahi et al. 2009). Comparatively, groundwater is less exposed to contamination than surface water. This reason underscores the reason, while it is more preferable to surface water. To man, groundwater has provided a great socioeconomic benefits including domestic and industrial uses, irrigation and tourism. In groundwater exploration, vertical electrical sounding (VES) employing Schlumberger electrode configuration is a common geophysical technique (Ezeh and Ugwu 2010; Olawuyi and Abolarin 2013; George et al. 2011; Ibuot et al. 2013). This is because instrumentation is simple; field logistics are easy and straightforward, while the analysis of data is less tedious and economical (Zohdy et al. 1974; Ekine and Osobonye 1996). The resistivity method is aimed at measuring the potential differences on the surface due to the current flow within the ground. Since the mechanisms that control the fluid flow and electric current and conduction are generally governed by the same physical parameters and lithological attributes, the hydraulic and electrical conductivities are dependent on each other (George et al. 2015). The need for water in the study area has aroused interest in the use of groundwater due to lack of surface water both saline and fraught with coliform. Most of the hand-dug

2 436 Int. J. Environ. Sci. Technol. (2016) 13: wells or drilled boreholes have been done without any preliminary geophysical investigations. This has resulted to failures of some boreholes and contamination of water which has resulted to various waterborne diseases. one of the surface water is as hygienic or as economical for exploitation as the groundwater (Singh 2007). Groundwater is recommended for its natural microbiological quality and its general chemical quality for most uses (McDonald et al. 2002). The guinea worm infestations in some parts of Ebonyi State are attributed to ignorance and lack of safe drinking water (Okoronkwo 2003). The people of Ezza lack functional boreholes and depend only on ponds and other existing contaminated and coliform-stricken surface sources which are open to physical, chemical and microbial contaminations. The inhabitants of the area had suffered severely from the outbreak of guinea worm, and they trek long distances in search of water, especially during dry season. The area has argillaceous minerals that seem to act as a protective cover to the underlying layers. The intrusions that gave rise to the existence of rocks and minerals during the Santonian uplift account for several fractures within the shale. These fractures contain water, serving as aquifer. Groundwater flow in fractured aquifers is very complicated, and accuracy in estimation of the hydraulic parameters depends on the hydraulic behavior in particular fractures, which is site specific (Singh 2005). The determination of aquifer characteristics involves the analysis and interpretation of soil and water samples of drilled boreholes, but due to the fact that these tests are capital and labor intensive, a noninvasive geoelectrical method (vertical electrical sounding) is used as an alternative to pumping tests. This paper attempts to evaluate aquifer potential, geoelectric and hydraulic parameters in the study area. Location and geology of the study area The study area lies between latitudes and of the equator and longitudes E and E of the Greenwich Meridian (Fig. 1). The area covers about 246 square kilometers and lies in the southeastern part of igeria. The study area belongs to the Asu River group shales. The sediments of the Asu River group which was formed during the Albian times were folded into open northeast trend known as Abakaliki Anticlinorium (Reyment 1965). The Asu River group is overlain by succession of shales, siltstones and sandstones, with shallow marine fauna, and is estimated to have a maximum thickness of about 200 m. There are some mineral intrusions which may have contributed to its numerous fractures. The geological survey around the area reveals that the location is part of the Ebonyi Formation that overlies the Abakaliki siltstone and sandstone previously referred to in the literature as Unknown Formation (Reyment 1965). It is now referred to as Ebonyi Formation (Agumanu 1989). The Formation underlies a gentle undulating terrain in tezi Ezamgbo area and southward to Amagu-Agba in Ebonyi State. The Ebonyi River and its tributaries (Akaduru, ramura and Isumutu Rivers) form the major drainage system in the area. The formation is divided into three units from top to bottom. The upper siltstone shale sequence is exposed at Amagu-Agba village. It consists essentially of rapidly alternating siltstone and silty shale with occasional thin sandstone beds. The middle limestone siltstone sequence unit outcrops at a quarry, 2 km from Ekemoha to Agba road junction. It consists of minor sandstone, siltstone, limestone and shale. Lastly, the lower mudstone shale sequence exposed at Umuezeoke, along drainage cut by River Akaduru. This sequence is grayish, occasionally flesh-colored and bedded with dark micaceous steaks. The study area has elevation between 57 and 89 m above sea level. Marshy conditions of lower elevation that also exist within the area are noted for rice production in the area. Most of the streams existing in the area are seasonal. The seasonal rivers which are active during the rainy seasons have the major drainage, the Ebonyi River, which flows to the Cross River, some distance to the south near Afikpo. The mudstones are highly weathered on the top. Significant groundwater is only found where the mudstone and shale are highly fractured. Materials and methods Twelve (12) vertical electrical soundings (VES) were carried out in the study area using OHMEGA SAS1000 Terrameter with its accessories. The Schlumberger electrode array was employed for each VES profile with half current ( AB 2 ) electrode separation of 150 m and half potential (M 2 ) electrode separation of 15 m. To reduce the field data to their equivalent geological models, both manual and computer modeling techniques were employed (Zohdy et al. 1974; Akpan et al. 2009). The observed field data were converted to apparent resistivity (q a ) values using Eq. (1): " AB 2 M 2 # 2 2 q a ¼ p R a ð1þ M The manual procedure was done by plotting a graph of apparent resistivity against half-electrode spacing using a bi-logarithmic graph, and the curves generated were smoothened to remove the effects of lateral inhomogeneities and other forms of noisy signatures (Bhattacharya and Patra 1968; Chakravarthi et al. 2007).

3 Int. J. Environ. Sci. Technol. (2016) 13: Fig. 1 Location map of the study area showing VES points A conventional manual curve procedure using master curves and auxiliary charts (Orellana and Mooney 1966) was used to quantitatively interpret the smoothened curves in terms of true resistivity and thickness. The parameters obtained from curve matching were used by the computer software RESOUD as input data for the computer iterative modeling. Figure 2 shows typical geologic models obtained along with their correlations with nearby boreholes. For the interpretation and understanding of the geologic model, some parameters related to different combination of thickness and resistivity of geoelectrical layer are necessary (Zohdy et al. 1974; Maillet 1947). These are the Dar Zarrouk parameters: longitudinal conductance (S) and transverse resistance (T), which are, respectively, given by: S ¼ h q T ¼ hq ð2þ ð3þ where h is the layer thickness in meters and q is the electrical resistivity of the layer in ohmmeters. Since the area has the same characteristics as the one studied by (Heigold et al. 1979), the hydraulic conductivity (K) was estimated using K ¼ 386:40 R :93283 rw ð4þ where R rw is the resistivity of the aquifer. The aquifer transmissivity (T r ) was estimated using the relation (iwas and Singhal 1981): T r ¼ KrT ¼ KS r ¼ Kh ð5þ where r is the electrical conductivity (inverse of resistivity), S is the longitudinal conductance and T is the transverse resistance. Equations (4) and (5) were used in this study to determine the hydraulic conductivity and transmissivity of aquifers, which depends on lithology and salinity of an area. The reflection coefficient (Rc) and fractured contrast (Fc) shown in Table 2 were calculated using Eqs. 6 and 7, respectively; Rc ¼ q n q n 1 ð6þ q n þ q n 1 Fc ¼ q n ð7þ q n 1 where q n is the resistivity of the nth layer and q n-1 is the layer resistivity overlying the nth layer.

4 438 Int. J. Environ. Sci. Technol. (2016) 13: Fig. 2 Samples of 1-D derived modeled VES curves correlating with nearby borehole Results and discussion The results of the geoelectric sounding data revealed four to six geoelectric layers with varying intrafacies and interfacies changes (Table 1). The third and fourth layers constitute the aquiferous zones except VES 9 (Ohaccara- diegu-ohaccara) where the aquifer is the fifth layer. Eight geoelectric curve types were identified and grouped as: QQH (25 %), 16.7 % of QHK and QHA, while 8.3 % represents QQQ, HAK, KHK, HKH and QQ. The first layer resistivity and thickness range between Xm and m, respectively, and consists of the top lateritic sand. The second layer has resistivity and thickness range of Xm and m, respectively. The third layer has a resistivity range of Xm and thickness range of m with that of VES 10 (diegu-ekka-onunwode diegu) undefined. This layer harbors most of the aquifers in the study area. The fifth layer with resistivity range of Xm has infinite thickness in most of the study area. Resistivity of the aquifer layers is shown in Table 2 with an average value of Xm. The variability of its resistivity values is shown in Fig. 3. It reveals that the greater part of the study area is characterized by low aquifer resistivity values \500 Xm. Areas with resistivity values \100 Xm indicate argillaceous formation facies which may lower the aquifer potentials, but areas with resistivity values [100 Xm (Ogboji-Eguo-Ugwu and Ohaccara-diagu-ohaccara) indicate clay sand sequence, and this is an indication of good aquifer formation. Figure 3 shows that high resistivity is obtainable in the southeastern part of the study area. Figure 4 is an isopach contour map showing the variation in aquifer thickness in the study area. The values of aquifer thickness range from 34.1 to m at Adiagu- Oguji wudor and diegu-ogboji-ukwu Akpara, respectively, with an average value of m. The map shows the increase in aquifer thickness from the eastern part to the western part of the study area. The distribution of aquifer longitudinal conductance (S) is shown in Fig. 5. The values range from at Ogboji-Egwu-Ugwu to 4.6 mhos at Ekka integrated primary school, Ekka, with an average value of mhos. From Table 2, the longitudinal conductance values aided in classification of the study area into weak, moderate and good aquifer protective capacity. VES 8 (Ogboji-Eguo-Ugwu) was classified as weak. VES 1, 7 and 9 were classified as moderate, while the rest have good aquifer protective capacity, according to Henriet (1976) classification. VES 8 is vulnerable to

5 Int. J. Environ. Sci. Technol. (2016) 13: Table 1 Summary of results of interpreted layer parameters Layer resistivity Layer thickness Curve type Latitude ( ) VES station name Longitude ( ) VES station h 6 h 5 h 4 h 3 h 2 h 1 q 6 q 5 q 4 q 3 q 2 q 1 1 Adiagu-Oguji wudor E ? QHK 2 SEkka Town Hall E ? QQQ 3 komoro-omuzor E ? QHA 4 diegu-ogboji E ? HAK 5 Umundiegu-Ohaike E ? QQH 6 Udenyi-Azuakparata E ? QQH 7 Inyere-gangbo E ? QHK 8 Ogboji-Eguo-Ugwu E ? KHK 9 Ohaccara-andiegu E ? HKH Ohaccara 10 diegu-ekka-onunwode E ? QQ 11 Ekka Integrated Sc E ? QHA 12 Ohaugo Pr Sc Ekka E ? QQH contamination from leachate. Areas that are moderate are less vulnerable, while areas with good protective capacity are not vulnerable to contamination from leachate infiltration. The average value of longitudinal conductance shows that the study area has good protective capacity. The weak protective capacity zone is associated with arenaceous material content compared to the areas with moderate and good protective capacity which the underlain aquifers are protected by the overlying argillaceous minerals. The transverse resistance (T) of the study area varies from to 38,808 Xm 2 with an average value of Xm 2. The distribution of the aquifer transverse resistance is shown in Fig. 6. Maximum transverse resistance is observed in the western part of the area and part of the southeastern area. This indicates that the western part of the area has high thickness as can be seen from the isopach map, and it can be assumed that these areas may likely have high transmissivity and high yield of aquifer units. The aquifer hydraulic conductivity (K) with an average value of m/day ranges from to m/day. The high range of hydraulic conductivity of aquifers is due to the heterogeneity of the aquifer sand repository, a condition responsible for wide range in hydraulic conductivity (George et al. 2015). Hydraulic conductivity is a measure of the ease with which a fluid will pass through a medium (Heigold et al. 1979). The distribution of hydraulic conductivity is shown in Fig. 7. It revealed that the greater part of the study area has low hydraulic conductivity values, indicating that groundwater flow in the area is not simple but complex because of the geologic controls of the confined aquifers. The transmissivity contour map is shown in Fig. 8. The transmissivity (T r ) value range from to m 2 /day (Table 2). Transmissivity values increase at the extremes of the study area, leaving the middle part with low values B600 m 2 /day. Areas with high transmissivity values can be identified as areas of high waterbearing potential, and aquifer materials are highly permeable to fluid movement. The average transmissivity value of m 2 /day indicates that the area has moderate-tohigh aquifer potential. The reflection coefficient (Rc) and the fracture contrast (Fc) are represented in contour maps (Figs. 9 and 10). Though the highest value is found at the S part of the study area, reflection coefficient (Rc) is observed to be high at the western part of the study area. The values range from to with an average value of The fracture contrast ranges from to with the average value of The highest value of the fracture contrast is obtainable at the southeastern part of the study area. Lower values of fracture contrast (Fc) are

6 440 Int. J. Environ. Sci. Technol. (2016) 13: Table 2 Summary of aquifer electrical and hydraulic parameters VES q b h S (mhom) T (Xm 2 ) K (m/day) T r (m 2 /day) Rc Fc Adiagu-Oguji wudor Ekk Town Hall Azugwu komoro-omuzor Ogbo Ojiovu diegu-ogboji-ukwu Akpara , Umundiegu-Ohaike Udenyi-Azuakparata Inyere-gangbo wakpa Umobi Ogboji-Eguo-Ugwu , Ohaccara-diegu-Ohaccara diegu-ekka-onunwode diegu Ekka Integrated Pri. Sch Ekka Ohaugo Pri Sch Ekka Average Fig. 3 Contour map of the study area showing aquifer resistivity in ohmmeters Resis vity (Wm) Fig. 4 Isopach contour map showing the variation of thickness in the study area Thickness

7 Int. J. Environ. Sci. Technol. (2016) 13: Fig. 5 Contour map showing the distribution of longitudinal conductance in the study area Longitudinal conductance (W -1 ) Fig. 6 Contour map showing the distribution of transverse resistance in the study area Transverse resistance (Wm 2 ) Fig. 7 Map showing the distribution of aquifer hydraulic conductivity Hydraulic conduc vity (m/day)

8 442 Int. J. Environ. Sci. Technol. (2016) 13: Fig. 8 Contour map showing the distribution of aquifer transmissivity in the study area Transmissivity (m 2 /day) Fig. 9 Map showing the variation in reflection coefficient map Reflec on coefficient Fig. 10 Map showing the distribution of fracture contrast in the study area Fracture contrast

9 Int. J. Environ. Sci. Technol. (2016) 13: found in between the east and south part of the study area. It can be said that the reflection coefficient value \0.9 and resistivity contrast value \19 may indicate high-density water-filled fractures (Olayinka et al. 2000). Based on the spread of Rc and Fc in Figs. 9 and 10, respectively, the part of the mapped area in between the eastern and western parts of the study area is characterized with high-density water-filled fractures. Conclusion In this paper, the electrical resistivity sounding method was used to explore the study area and to evaluate the geoelectrohydraulic parameters. The results provide data on aquifer electrical and hydraulic parameters which included the longitudinal conductance, transverse resistance, hydraulic conductivity, transmissivity, reflection coefficient and fracture contrast. These parameters were used to generate different contour maps. The frequency of curve types indicates that the area is dominated by QQH curve type with 25 and 16.7 % of QHK and QHA, respectively, while the rest have 8.3 % each. The result revealed that areas with high transverse resistance values may give high aquifer yield; the study area has good aquifer protective capacity due to the argillaceous overlying clay materials and also moderate-to-high aquifer potential. The results also show that the shallow aquifers characterized with wide ranges of hydraulic conductivity caused by heterogeneous facies change in the area can be located in the fine-sand facies underlying the clayey formation. Acknowledgments Authors are grateful to Dr. J. U. Chukudebelu, Department of Physics and Astronomy, University of igeria, sukka and Solomon Offiah, ational Centre for Energy Research and Development, University of igeria, sukka, for their contributions and encouragement. 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